Yes, SmartRhythm causes a higher battery usage and you will need to charge your watch more frequently. This is because SmartRhythm evaluates heart rate data continually and thus requires the Apple Watch’s to operate as a "Motion and Fitness" activity to collect heart rate date from the Apple Watch when SmartRhythm is on and this increases battery usage. Our internal testing showed that with SmartRhythm turned on, you should expect about 16 hours or so of battery life from your series 1 or newer watch.
We recommend charging your Watch overnight or for an hour in the evening and morning if you plan to wear your Watch while sleeping. Some users take the time to charge their watch during showers or while brushing their teeth to extend battery life.
The first generation Apple Watch is not supported because it has a smaller battery and will only last about 5 hours while using SmartRhythm.